1. Field of the Invention
The invention relates to obtaining hydrofluoric acid as a byproduct of a process of deriving adjusted phosphorous compounds from naturally occurring phosphate rock.
2. Background Information
Mined phosphate rock is commonly defluorinated by heating the rock to an elevated temperature in a kiln or fluidized bed reactor. For simplicity of description, an exemplar kiln process will be described herein, but those skilled in the art will recognize that other defluorination processes can be used and that those processes would also deliver a fluorine-rich flue gas. In one version of the defluorination process a kiln is used that is about two hundred fifty feet long with an internal diameter of about six feet. This generally horizontal tubular kiln slopes downward from an infeed end with a slope of about three feet for each twenty feet of length. Crushed and chemically adjusted phosphate rock is fed into the kiln at the upper, low temperature, end and moves under the influence of gravity to the lower end where the temperature is commonly on the order of twenty six hundred degrees Fahrenheit. At these high temperatures the bound fluorine in the rock is liberated and the hot fluorine-rich gas flows back up the kiln and is exhausted at the upper end thereof at a temperature of about thirteen hundred degrees Fahrenheit. The exhausted gas is accompanied by a significant amount of phosphate-rich dust.
It is known in the prior art to capture and dispose of fluorine compounds contained in the hot exhausted gas mixture by conveying the gas mixture through a spray of water, collecting the liquid effluent and neutralizing the effluent liquid with limestone so as to tie the fluorine up as calcium fluoride. It is also known in the prior art to collect the effluent liquid, which is commonly less than 6% HF by weight, and recover a fraction of the fluoride values therefrom. Only about half of the available HF is recovered in these processes. The rest must be neutralized and the precipitate from the neutralization process dumped. Moreover, because the hot exhausted gas mixture contains a large amount of entrained phosphate rock dust, the product from prior art recovery processes is contaminated with phosphorous.